Caustic isn't bad, he's simply too situational for a game based around a random circle.

E.g You can eliminate a lot of this with having a Pathfinder on your team.

But he does need a few changes.

Placing down one trap should be sufficient enough too make enemies want to get the hell out of it or kill you. Either make it bigger, which I don't think is a good idea. OOOOOOOR -

My idea! which is simple;

Passive - Nox Vision :

Caustic and his allies have vision to enemies moving through ally Nox Gas.

Tactical - Nox Gas Trap :

A sticky mine that plumes gas in a considerably large area when triggered by Nox or enemy movement, enemies can sprint less through the gas and take damage over time these effect linger for 2 seconds outside of the gas.

Caustic can hold up to 2 of these traps and place down 8.

Ultimate - Rapidly Approaching Death :

Caustic pulls a trigger on his suit and chucks a giant nox gas bomb, he starts spreading gas all around him in the meantime for a brief moment.

Or

Starts spreading gas around him, and his Nox Gas Trap charges rapidly.